Magnetic random access memory (MRAM) (also known as magneto-resistive random access memory) is non-volatile random access memory available as an alternative to dynamic random access memory (DRAM) and static random access memory (SRAM). Data in MRAM is stored by magnetic storage elements formed from ferromagnetic layers having a non-magnetic barrier layer between the ferromagnetic layers in a configuration known as a magnetic tunnel junction (MTJ). A memory device can include a plurality of MTJ structures arranged in, for example, a grid.
A first one of the ferromagnetic layers (also referred to as a “fixed layer”) has a fixed magnetic moment or polarity, and second one of the ferromagnetic layers (also referred to as a “free layer”) has a variable magnetic moment or polarity which is able to be switched between same and opposite directions with respect to the magnetization direction of the fixed layer.
Same and opposite magnetic alignment with respect to the fixed layer can be referred to as “parallel” and “antiparallel” states, respectively. When the two ferromagnetic layers are aligned parallel, the resistance is considered to be “low,” and when the two ferromagnetic layers are aligned anti-parallel, the resistance is considered to be “high.” Changing writing current polarities changes the free layer magnetization between parallel and anti-parallel alignment with respect to the fixed layer, which respectively correspond to low resistance “0” and high resistance “1” states. Detecting changes in resistance permits an MRAM device to provide information stored in a magnetic memory element; in other words, perform a read operation.
Spin-transfer torque (STT) refers to an effect in which the orientation of a magnetic layer in an MTJ can be modified using a spin-polarized current. Charge carriers, such as electrons, have an intrinsic spin or angular momentum. An unpolarized electric current includes the same number of electrons having opposite spin from each other, and a spin polarized current includes more electrons of one spin than an opposite spin. When a current enters the fixed layer, it is polarized by reflection or transmission through the fixed layer. If the spin-polarized current is then directed into the free layer, the angular momentum can be transferred to the free layer, changing its orientation. Using the phenomenon of STT, an STT-MRAM can utilize a lower switching current than conventional MRAM, and delivers the high performance of DRAM and SRAM, at lower power and lower cost.
Magnetic anisotropy refers to the directional dependence of a material's magnetic properties. For example, the magnetic moment of magnetically anisotropic materials will tend to align with an energetically favorable direction of spontaneous magnetization, referred to an “easy axis.” Improving the free layer perpendicular magnetic anisotropy (PMA), and the thermal stability of the free layer is one of the key challenges for STT-MRAM technology.